The OSS4 drivers for Intel HDA codecs support high quality playback (excluse mode). You may not need to purchase a professinal soundcard, such as LynxTWO _http://manuals.opensound.com/usersguide/lynxtwo.html

The cheapest Sennheiser headphones might be sufficient for the purpose, for example:Sennheiser HD 201 – 22 Euro (warning: they are very loud with OSS4, you have to reduce the volume level to minimum).You may also want to try the most expensive headphones for audiophiles.

WARNING: You may not hear any difference between HiRes and CD format with ALSA, simply because ALSA resamples everything with a low quality resampler. Since ALSA does not provide any sort of "exclusive mode", it should not be used for hearing tests. However, if an ALSA user believes that ALSA permits a sort of "exclusive mode", he may try the test for deafness with ALSA. If he does not hear the difference between HiRes audio and CD format, it may mean that ALSA is dangerous for health.

It seems that FreeBSD has a sort of "pseudo exclusive mode" named "bitperfect mode". The FreeBSD used are invited to believe that, when the "bitperfect mode" is enabled, "the pure sound stream will be fed directly to the hardware" without resampling or dsp processing.

dev.pcm.%d.bitperfect Enable or disable bitperfect mode. When enabled, channels will skip all dsp processing, such as channel matrixing, rate convert- ing and equalizing. The pure sound stream will be fed directly to the hardware. If VCHANs are enabled, the bitperfect mode will use the VCHAN format/rate as the definitive format/rate target. The recommended way to use bitperfect mode is to disable VCHANs and enable this sysctl. Default is disabled.

_https://www.freebsd.org/cgi/man.cgi?query=pcm&sektion=4

The Mac OS users may not need fool themselves with HiRes audio and hearing tests, because Mac OS does not support "exclusive mode". There is, however, a naive belief that certain third-party apps, such as "BitPerfect", may disable resampling (such beliefs and superstitions can hardly be verified with clear-cut tests):

Exclusive Mode aka Hog Mode : What really matters on a Mac OS is how the player takes control of the settings in core audio of you machine. Open up "Audio- and MIDI-settings" and show the audio window. Play files with different sample rates and bit-depth from iTunes and you will se that core audio doesn't change. That means a sample rate conversion is taking place. Now download and run an app like the BitPerfect and make the same test. You should now observe that the core settings are actively adapted for each and every song you play in iTunes. This is what players like Amarra Music Player, Pure Music and Audirvana also does; they take active control and optimize the core audio settings to avoid any local real-time sample rate conversion._http://www.2l.no/hires/index.html

What really matters is what the term "exclusive mode" actually means.The true meaning of the term is that certain software causes of sound distortions are somehow removed:1. any sort of software mixing and/or redirection to virtual mixer engines is disabled,2. sample rate/format conversions are disabled,3. any sort of dsp processing, such as equalizing, is disabled.

It seems certain sound systems, such as ALSA, FreeBSD pseudo-OSS, and MAC OS sound system, do not provide any possibility to disable their software mixers. Such "software mixers" are a sort of buggy software layer between "the pure sound stream" and "the hardware". This buggy software layer is one of the main causes of sound distortions.